1. Field of the Invention
The invention relates to an electric power steering device an electric power steering system that drive a motor based on a steering operation of a driver to generate a steering assist torque.
2. Description of Related Art
There is an electric power steering device that controls an energization amount of a motor in accordance with a steering torque applied to a steering wheel by a driver to generate a steering assist torque. In such an electric power steering device, in the case where the driver performs a steering operation strongly, a large current flows from a vehicle-mounted power supply to the motor. When the vehicle-mounted power supply is degraded, an output voltage (power supply voltage) of the vehicle-mounted power supply decreases during energization of the motor. As the degree of degradation of the vehicle-mounted power supply is larger, the decrease of the power supply voltage to the current flowing to a load from the vehicle-mounted power supply becomes more significant. A control unit (referred to as an ECU) for controlling the motor includes electronic control components such as a microcomputer, a sensor, and the like. Accordingly, power supply within a specified voltage is required and, since the power is supplied from the vehicle-mounted power supply shared by the motor, it is necessary to suppress the decrease of the voltage of the vehicle-mounted power supply.
To cope with this, a technique for limiting steering assist in the case where the power supply voltage decreases is proposed conventionally. For example, Japanese Patent Application Publication No. 2005-193751 (JP 2005-193751 A) proposes a technique in which a current upper limit value as the upper limit value of the current caused to pass through the motor is decreased in the case where the power supply voltage decreases during the steering assist, and the current upper limit value is gradually returned to a normal value in the case where the power supply voltage recovers.
In the electric power steering device, basically, in the case where the steering assist is limited using the current upper limit value, the ECU calculates a current upper limit value Ilim according to, e.g., an upper limit characteristic shown in FIG. 8 in which a power supply voltage V is associated with the current upper limit value Ilim, and controls the energization of the motor within the range of the current upper limit value Ilim. Hysteresis is provided in the upper limit characteristic, and the relationship between the power supply voltage V and the current upper limit value Ilim in the case where the power supply voltage decreases is different from the relationship therebetween in the case where the power supply voltage increases. Specifically, when the power supply voltage V decreases and becomes lower than a decrease start voltage V1, the current upper limit value Ilim is set so as to decrease from a basic current upper limit value Ilim0 with a gradient α1 in response to the decrease of the power supply voltage V. In the case where the power supply voltage V becomes lower than a assist prohibition voltage V2, the current upper limit value Ilim is set to zero.
In the case where the power supply voltage V starts to increase, the current upper limit value Ilim is set so as to increase with a gradient α2 in response to the increase of the power supply voltage V after an increase amount of the power supply voltage V exceeds a hysteresis amount. For example, in the case where the power supply voltage V becomes lower than the assist prohibition voltage V2 and then starts to increase, the current upper limit value Ilim is maintained at zero until the power supply voltage V exceeds an increase start voltage V3 (>V2), and the current upper limit value Ilim is set so as to increase in response to the increase of the power supply voltage V after the power supply voltage V exceeds the increase start voltage V3.
The upper limit characteristic needs to be set such that upper limit limitation of the current (hereinafter referred to as current limitation) can be performed appropriately on constant factors for reduction of a power supply ability such as the degradation of the vehicle-mounted power supply. For example, the power supply ability is reduced in the case where an in-vehicle battery is degraded, and hence, when the current limitation is weakened (the current upper limit value is increased) immediately after the power supply voltage starts to increase due to the current limitation, a hunting phenomenon in which the power supply voltage decreases and the current limitation is intensified again occurs due to its reaction. In response to this, the steering assist torque fluctuates, and a steering operation feeling deteriorates. To cope with this, the hysteresis is set in the upper limit characteristic. In addition, it is necessary to set a gentle gradient as the gradient α2 such that a driver does not operate a steering wheel excessively with a sudden increase of the steering assist.
By executing the current limitation by using the upper limit characteristic set in the manner described above, even in the case where the vehicle-mounted power supply is degraded, it is possible to maintain the power supply voltage of the vehicle-mounted power supply within a specified range as much as possible.